Journal of Conference Abstracts

Linking the Thermal and Chemical Composition of Hydrothermal Fluids with Vent Plume Processes for Cadmium, Copper, Lead and Zinc

John H. Trefry Department of Oceanography, Florida Institute of Technology, Melbourne, FL 32901, USA

jtrefry@fit.edu

Simone Metz Department of Oceanography, Florida Institute of Technology, Melbourne, FL 32901, USA

Robert P. Trocine Department of Oceanography, Florida Institute of Technology, Melbourne, FL 32901, USA

David A. Butterfield NOAA, Pacific Marine Environmental Lab., 7600 Sand Point Way, Seattle, WA 98115, USA

Gary J. Massoth NOAA, Pacific Marine Environmental Lab., 7600 Sand Point Way, Seattle, WA 98115, USA

Richard A. Feely NOAA, Pacific Marine Environmental Lab., 7600 Sand Point Way, Seattle, WA 98115, USA

The importance of hydrothermal fluids to marine geochemical cycles for trace metals is partly controlled by reactions occurring in buoyant and neutrally-buoyant vent plumes. These plume reactions are in turn linked to the thermal and chemical composition of the hydrothermal fluid. We have collected and analyzed hydrothermal fluid and vent plume samples from the TAG Hydrothermal Field at 26°N on the Mid-Atlantic Ridge and the southern Juan de Fuca Ridge (SJFR) in the northeast Pacific Ocean. Our work focuses on Cd, Cu, Pb and Zn, four metals that are greatly enriched in vent fluids relative to seawater.

Temperature is the primary variable controlling concentrations of Cu in hydrothermal fluids. At the TAG Hydrothermal Field, vent fluids with temperatures of 363°C had Cu values as high as 108 mmol/kg relative to Cu levels of 17.5 mmol/kg at temperatures of 332°C on the SJFR. As temperatures decrease, Cu concentrations decrease sharply to values of 1.1 ± 0.3 mmol/kg between 246° and 280°C. By comparison, values for Cd, Pb and Zn in vent fluids are more controlled by chlorinity than temperature in the 246° to 363°C range. For example, a linear relationship between Cl and Zn (r² = 0.77) was observed for high-temperature (>240°C) solutions from both TAG (Cl = 629 mmol/kg and Zn = 50 mmol/kg) and the SJFR (Pipe Organ vent, Cl = 1245 mmol/kg and Zn = 520 mmol/kg). This overall trend for vent fluids results from postulated mixing of a brine phase, an evolved seawater phase and a vapor phase with chloride serving as an indicator of the relative inputs of each component, especially metal-rich brine. When Zn/Cl relationships for individual vent areas are considered, the value for r² is greater than 0.90, implying that observed differences may result from differences in brine composition. Alternatively, hydrothermal fluids also may dissolve additional Zn and other metals during ascent to the seafloor through sphalerite-lined conduits. For example, fluids from sphalerite-rich "onion domes" at TAG (252°C, Cl = 625 mmol/kg) had Zn concentrations in excess of 500 mmol/kg. Thus, when temperature is not a dominant controlling factor (e.g., Cu), major element composition and sub-seafloor metal refining may be more important. Whatever combination of processes control Zn concentrations in vent fluids, we have learned that concentrations of Cd and Pb show strong linear relationships with Zn and are most likely regulated by the same processes. Metal/Zn ratios for fluids from divergent locations (TAG and SJFR) and different temperature and chemical regimes generally fit on the same line.

By combining reliable values for metals in vent fluids with concentrations of dissolved and particulate metals in buoyant plumes, we have been able to determine the fraction of vent-derived metal that survives passage in the dissolved form through the vent fluid/seawater mixing zone. Mass balances in the plumes are checked by comparing vent fluid data and approximate dilution factors (from dissolved Mn and Si) in the buoyant plume with direct plume measurements. Measured total Zn levels in the plume agree well (r² > 0.98) with values calculated from vent fluid dilution factors. Data for the TAG area and the SJFR show that ratios of Pb/Zn and Cu/Zn in plume particles are the same as in the vent fluid. When coupled with dissolved metal data showing no enrichment of either metal in the plume above 20 m, the results support quantitative removal of Cu, Pb and Zn from solution in the buoyant plume, thereby minimizing the role of vent fluids as net sources of these metals to seawater. In contrast, data for particulate Cd/Zn show excess Cd in plume particles. This extra Cd is balanced by negative anomalies as large as 0.8 nmol/kg for concentrations of dissolved Cd in the plume, indicating removal of vent-derived plus seawater Cd. Such observations imply that hydrothermal circulation could be a net sink for Cd.

Collectively, similar metal ratios are found for hydrothermal fluids and vent particles for a specific site; however, differences in Zn/Cu ratios among sites are observed. These differences are indirectly related to the temperature and composition of the source fluid. For example, Zn/Cu ratios show Pipe Organ vent (SJFR, 261°C)=70 > Vent 1 (SJFR, 332°C)=20 > TAG (363°C)=0.5. Thus, we may be able to analyze plume particles from a new area or even ancient sediments and determine the general temperature and chemical regime of the source fluid.